Single cell and stack structure for solid oxide fuel cell stacks

ABSTRACT

A single cell and stack structure for SOFC stacks is disclosed. The single cell consists of a fuel electrode, an electrolyte and an air electrode, with opposite two or four sides of said single cell is shaped while being bent downwardly, thus forming an electrode support type structure or a self-support (electrolyte support) type structure each having a reversed U-shaped cross-section. In the SOFC stack structure, electrode support type or self-support type single cells are gastightly stacked on a separating plate while being held on a plurality of sealing grooves sealed with sealant. In the SOFC stack, the fuel gas and the oxidizing gas are free from being mixed together due to an improved gas sealing structure. The SOFC stack is thus free from a stress due to a difference in coefficient of thermal expansion between the single cells and the separating plate at a raised or lowered temperature. When the SOFC stack is assembled, the bent support portions of the single cells are precisely seated in the sealing grooves of the separating plate. The single cells are thus stably held within the SOFC stack while lengthening the expected life span of the stack, improving the durability and the operational reliability of the stack, and allowing a user to more easily repair the stack when necessary.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates, in general, to solid oxide fuelcells and, more particularly, to a single cell for solid oxide fuel cellstacks, being shaped while being bent downwardly at opposite two or foursides of the cell to form an electrode support type structure or aself-support (electrolyte support) type structure each having a reversedU-shaped cross-section, the present invention also relating to a solidoxide fuel cell stack structure with such single cells being gastightlystacked on a separating plate at a plurality of sealing grooves of theplate sealed with sealant.

[0003] 2. Description of the Prior Art

[0004] As well known to those skilled in the art, fuel cells aredesigned to accomplish a smooth flow of reaction gases to two electrodes(i.e., anode and cathode), to bring the two electrodes into electriccontact with an electrolyte substrate, and to accomplish a gastightsealing effect between the reaction gases. The fuel cells thus induce anionic conduction from the electrodes toward the dense electrolytesubstrate and create an electrochemical reaction in the electrodes,thereby forming electromotive force and finally generating electricpower using the electromotive force.

[0005] In recent years, solid oxide fuel cells (hereinbelow, referred tosimply as “SOFC”) have been proposed and used while being so-called “athird generation fuel cell”. In such an SOFC, a thermochemically stablemetal oxide is used as the material of an electrolyte substrate, with afuel electrode (anode) and an air electrode (cathode) being respectivelyattached to both (lower and upper) sides of the electrolyte substrate.Such an SOFC somewhat freely uses a variety of fuel gases, such as H₂,CH₄, CH₃OH, etc., without reforming the fuel gases, and uses air oroxygen as an oxidant, thus effectively accomplishing a highly efficientand low pollution power plant.

[0006] A conventional SOFC stack consists of a fuel electrode (anode)(Ni-YSZ cermet), an electrolyte [doped zirconia (ZrO₂+8Y₂O₃), dopedceria (CeO₂), doped bismuth oxide (Bi₂O₃), doped perovskite], an airelectrode (LaSrMnO₃), a separating plate or an interconnector(Cr—5Fe—1Y₂O₃, Ni-based metal, LaSrCrO₃), a current collector, and asealant (glass or glass-ceramic). The above-mentioned elements areassembled into a desired SOFC stack. The SOFC stack is also assembledwith peripheral equipment, thus accomplishing a desired power generatingsystem.

[0007] Such an SOFC stack includes a plurality of single cells, eachconsisting of an electrolyte substrate with a fuel electrode as anegative electrode (anode) and an air electrode as a positive electrode(cathode) being attached to both sides of the electrolyte. In order toeffectively create a desired electrochemical reaction in the twoelectrodes, the electrodes each preferably have a porous structure. Inaddition, the electrolyte substrate, or the intermediate layer of thesingle cell, preferably has a dense structure which does not allow fuelgas or oxidizing gas to permeate into the electrolyte or to be mixedtogether.

[0008] When such single cells are stacked into a desired SOFC stack, thesingle cells are positioned between two separating plates. In such acase, it is necessary to form a desired gastight sealing structure usinga sealant, such as glass or glass-ceramic, within the stack so as toprevent two different gases from being mixed together while flowingalong opposite gas channels of the separating plates. It is alsonecessary to design the SOFC stack to allow a smooth gas supply for theopposite electrodes of each single cell. In addition, an insulatinglayer or an insulating plate, made of a sealing and insulating material,is provided on an area of the upper separating plate, with the areabeing free from the single cells.

[0009] Conventionally, the SOFCs are classified into three types, suchas a tubular type, a planar type and a monolithic type. Of the threetypes, the tubular type SOFC is the well-known type SOFC. However, sucha tubular type SOFC is problematic in that it is very difficult toproduce and is less likely to be practically used.

[0010] A known method of producing such a tubular type SOFC may bereferred to a Minh's report (N. Q. Minh, J. Am. Ceram. Soc., 76[3] p563-588, 1993). As disclosed in the above Minh's report, a porouselectrode support in the form of a tube having a length of 2 mm isprimarily produced through an extrusion process. Thereafter, a porouselectrode layer is formed on the porous tubular support through a slurrycoating process. In addition, both a desired electrolyte layer and adesired interconnector are formed through an EVD process(electrochemical vapor deposition process), thus producing a desiredtubular type SOFC. The tubular type SOFC is somewhat advantageous inthat it is easy and simple to accomplish both a desired gas sealingeffect and an interconnection of single cells while stacking the tubulartype SOFCs into an SOFC stack. However, the tubular type SOFC isproblematic in that it has a low power density in comparison with theplanar type SOFC or the monolithic type SOFC. In addition, it isnecessary to enlarge the size and volume of EVD equipment in proportionto the length of a desired tubular type SOFC. This finally forces theEVD equipment to be large-sized and increases the equipment cost.Furthermore, a multi-step process has to be used for producing such atubular type SOFC, thus increasing the production cost of single cells.Therefore, the tubular type SOFC will be less likely to be practicallyused.

[0011] Different from the tubular type SOFC and the monolithic typeSOFC, the planar type SOFC is advantageous in that the electrolyte thinsubstrate having a thickness of 200 μm may be made of inexpensiveconventional ceramic, thus being suitable for production in commercialquantity. Such a planar type SOFC also effectively improves the powerdensity to an extent which cannot be expected from the tubular type SOFCor the monolithic type SOFC due to their structural disadvantages. Inthis regard, such planar type SOFCs rather than the tubular type SOFCsor the monolithic type SOFCs have been actively studied and developedrecently.

[0012] Such planar type SOFCs are conventionally classified intoelectrode support type SOFCs and self-support (or electrolyte support)type SOFCs in accordance with the electrolyte, the electrode or thematerial. Of the two types, the self-support type (or electrolytesupport type) SOFCs are more widely used rather than the electrodesupport type SOFCs. As shown in FIG. 1a, such a self-support type SOFCis produced by coating a positive electrode (cathode) layer and anegative electrode (anode) layer, each having a thickness of several tenmicrometers, on both sides of an electrolyte substrate having athickness of 200 μm. A known method of producing such a electrodesupport type SOFC may be referred to a Souza's report (S. de. Souza, J.Electrochem. Soc., 144[3] L35-L37, 1997). As disclosed in the aboveSouza's report, an electrolyte thin layer having a thickness of 20 μm isformed on a porous electrode support having a thickness of 1˜2 mm, thusforming a desired electrode support type SOFC having a highly improvedelectric performance. When an SOFC stack is produced using suchelectrode support type single cells, it is possible to preferably reducethe operational temperature of the SOFC stack from 1,000° C. to about800° C. Therefore, the planar type SOFCs have been actively studied anddeveloped recently to provide improved electrode support type SOFCs.

[0013] In order to accomplish the recent trend of high power capacity ofSOFC stacks, it is necessary to assemble an SOFC stack having anenlarged area and this forces the area of each single cell of the stackto be enlarged. However, the known technique of producing ceramic thinplates only provides a square electrolyte plate having a dimension ofabout 10×10 cm or about 20×20 cm, the electrolyte plate being used as anelectrode support plate. Therefore, as disclosed in a Blum's report (L.Blum et al, Proceedings of the 4th Int. Symp. On SOFC, Vol 4, p 163-172,1995), a grid array SOFC stack, having a desired size larger than thatof each single cell, is preferably proposed to be used as a planar typeSOFC stack. In such a grid array SOFC stack, a plurality of single cellshaving a size smaller than that of a separating plate are arrayed inparallel on the separating plate while accomplishing a highly gassealing effect as shown in FIG. 1b. However, such a highly gas sealingeffect is very difficult to be accomplished during the process ofproducing the grid array stack, and so the requirement for the highlygas sealing effect stands in the way of practical use of such planartype SOFCs. The requirement for the highly gas sealing effect of theplanar type SOFC stacks is very important since it directly determinesthe durability and expected life span of such stacks.

SUMMARY OF THE INVENTION

[0014] Accordingly, the present invention has been made keeping in mindthe above problems occurring in the prior art, and an object of thepresent invention is to provide a single cell for SOFC stacks, is shapedwhile being bent downwardly at opposite two or four sides of the cell toform an electrode support type structure or a self-support (electorlytesupport) type structure each having a reversed U-shaped cross-section.

[0015] Another object of the present invention is to provide an SOFCstack structure, with electrode support type or self-support type singlecells being gastightly stacked on a separating plate while being held ona plurality of sealing grooves sealed with sealant.

[0016] In order to accomplish the above object, the present inventionprovides a single cell for SOFC stacks, comprising a fuel electrode, anelectrolyte and an air electrode, which is shaped while being bentdownwardly at opposite two or four sides of the cell to form anelectrode support type single cell or a self-support type single celleach having a reversed U-shaped cross-section.

[0017] The present invention also provides an SOFC stack structure, withelectrode support type or self-support (electrolyte support) type singlecells being gastightly stacked on a separating plate while being held ona plurality of sealing grooves sealed with sealant.

[0018] In the SOFC stack of this invention, the fuel gas and theoxidizing gas are free from being mixed together due to an improved gassealing structure. The SOFC stack is thus free from the stress due to adifference in coefficient of thermal expansion between the single cellsand the separating plate when the temperature of the stack is raised orlowered. Since the sealant is stably kept within the sealing groovesregardless of an environmental change, the SOFC stack is stably operatedwithout being affected in performance when the temperature of the stackis raised or lowered. When the SOFC stack is assembled, the bent supportportions of the single cells are precisely seated in the sealing groovesof the separating plate. The single cells are thus stably held withinthe SOFC stack irrespective of external impact or thermal stress. Thisfinally lengthens the expected life span of the SOFC stack, improves thedurability and the operational reliability of the stack, and allows auser to more easily repair the stack when necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0020]FIG. 1a is an exploded perspective view, schematically showing theconstruction of a conventional simple planar, self-support (elctrolytesupport) type single cell for SOFC stacks;

[0021]FIG. 1b is an exploded perspective view, schematically showing theconstruction of a grid array SOFC stack with a plurality of conventionalsingle cells of FIG. 1a;

[0022]FIG. 2a is a view, schematically showing the construction of afuel electrode (anode) support type single cell for SOFC stacks inaccordance with the primary embodiment of the present invention, withthe four sides of the cell being bent downwardly to form a reversedU-shaped cross-section of the cell;

[0023]FIG. 2b is a view, schematically showing the construction of anair electrode (cathode) support type single cell for SOFC stacks inaccordance with the second embodiment of this invention, with the foursides of the cell being bent downwardly to form a reversed U-shapedcross-section of the cell;

[0024]FIG. 2c is a view, schematically showing the construction of aself-support (electrolyte support) type single cell for SOFC stacks inaccordance with the third embodiment of this invention, with the foursides of the cell being bent downwardly to form a reversed U-shapedcross-section of the cell;

[0025]FIG. 3 is an exploded perspective view, schematically showing theconstruction of a grid array SOFC stack assembled using a plurality offour-side bent single cells of this invention;

[0026]FIG. 4 is a sectional view, schematically showing the constructionof a grid array SOFC stack assembled using the fuel electrode (anode)support type single cells of FIG. 2a;

[0027]FIG. 5 is a sectional view, schematically showing the constructionof a grid array SOFC stack assembled using the air electrode (cathode)support type single cells of FIG. 2b;

[0028]FIG. 6 is a sectional view, schematically showing the constructionof a grid array SOFC stack assembled using the self-support (electrolytesupport) type single cells of FIG. 2c;

[0029]FIG. 7a is a view, schematically showing the construction of afuel electrode (anode) support type single cell for SOFC stacks inaccordance with the fourth embodiment of the present invention, withopposite two sides of the cell being bent downwardly to form a reversedU-shaped cross-section of the cell;

[0030]FIG. 7b is a view, schematically showing the construction of anair electrode (cathode) support type single cell for SOFC stacks inaccordance with the fifth embodiment of this invention, with oppositetwo sides of the cell being bent downwardly to form a reversed U-shapedcross-section of the cell;

[0031]FIG. 7c is a view, schematically showing the construction of aself-support (electrolyte support) type single cell for SOFC stacks inaccordance with the sixth embodiment of this invention, with oppositetwo sides of the cell being bent downwardly to form a reversed U-shapedcross-section of the cell; and

[0032]FIG. 8 is an exploded perspective view, schematically showing theconstruction of a grid array SOFC stack assembled using a plurality oftwo-side bent single cells of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033]FIGS. 2a to 8 show single cells and stack structures for SOFCstacks in accordance with the preferred embodiments of the presentinvention. As shown in the drawings, the present invention provides afuel electrode (anode) support type single cell, an air electrode(cathode) support type single cell, and a self-support (electrolytesupport) type single cell for SOFC stacks. The present invention alsoprovides an SOFC stack structure having the above-mentioned singlecells.

[0034] In a fuel electrode support type single cell 4 of FIG. 2a, aporous fuel electrode 1, or an anode having a thickness of 1˜2 mm, isshaped while being bent downwardly at two or four opposite sides to forma reversed U-shaped cross-section, with a planar top portion 31, acorner portion 32 and a bent support portion 33. An electrolyte thinlayer 2, having a dense structure with a thickness of 10˜50 μm, iscoated on the top surface of the planar portion 31 and on the externalsurfaces of the corner and support portions 32 and 33 of the fuelelectrode 1. Thereafter, an air electrode, or a cathode having a porousstructure, is coated on the top planar surface of the electrolyte thinlayer 2. In a brief description, the fuel electrode support type singlecell 4 of FIG. 2a is a triple-layered single cell, with the electrolytemiddle layer 2, the fuel electrode lower layer 1 and the air electrodeupper layer 3. On the other hand, an air electrode support type singlecell 4 of FIG. 2b has a profile similar to that of the cell of FIG. 2a,with the positions of the fuel electrode layer 1 and the air electrodelayer 3 being reversed from those of the primary embodiment.

[0035] In a self-support (electrolyte support) type single cell 4 ofFIG. 2c, a dense structural electrolyte layer 2 having a thickness of100˜300 μm is shaped while being bent downwardly (at right angles) attwo or four opposite sides to form a reversed U-shaped cross-section. Aporous fuel electrode (anode) layer 1 is coated on the lower surface ofthe electrolyte layer 2, while an porous air electrode (cathode) layer 3is coated on the top surface of the electrolyte layer 2. Theself-support type single cell 4 has a triple-layered structure. Thesingle cells of FIGS. 2a to 2 c are used for producing an SOFC stackstructure 11 of FIG. 3. In the SOFC stack structure 11 of FIG. 3, aplurality of gas channels 6 are formed on a first separating plate 8,while a plurality of channel supports 7 are positioned on the gaschannels 6 with the single cells 4 being stacked on the channel supports7 in the stack 11. In order to assemble the bent support portions 33 ofthe single cells 4 with a second separating plate 9, a plurality ofsealing grooves 12 are formed on the second separating plate 9. Each ofthe sealing grooves 12 has a shape corresponding to the profile of thesupport portion 33 of each single cell 4. A sealant 5 is filled in thegrooves 12 prior to seating the bent support portions 33 of the singlecells 4 within the grooves 12. Thereafter, an insulating plate 10 islaid on the second separating plate 9 while surrounding the single cells4 and accomplishing a desired sealing effect between the insulatingplate 10 and the second separating plate 9, thus forming a desired SOFCstack structure 11. In the present invention, a plurality of stackstructures 11 with one being laid on top of another, thus forming adesired SOFC stack.

[0036] A better understanding of the present invention may be obtainedthrough the following examples which are set forth to illustrate, butare not to be construed as the limit of the present invention.

EXAMPLE 1 A Process of Producing a Fuel Electrode (Anode) Support TypeSingle Cell Which is Bent Downwardly at Four Sides, and a Process ofProducing an SOFC Stack Using Such Single Cells

[0037] In order to produce a desired fuel electrode (anode) support typesingle cell, NiO powder and yittria stabilized zirconia power(ZrO₂+8Y₂O₃, 8YSZ) were primarily mixed together at a weight ratio of50:50, thus preparing a powder mixture. Thereafter, 20% graphite powderwas mixed with the powder mixture, thus forming a starting material forsubstrates. A porous substrate, or a fuel electrode layer 1 having aporosity of 40%, was produced using the starting material. In such acase, the porous substrate 1 had a shape of FIG. 2a, was sinteredultimately into a square size of 55×55 mm, with a thickness of a squareportion 31 of 1˜2 mm, an outside height of a bent support portion 33 ofabout 3 mm, an inside height of the bent support portion 33 of about 1˜2mm, and a thickness of the bent support portion 33 of about 1˜2 mm.

[0038] Thereafter, an electrolyte, selected from doped zirconia (ZrO₂),doped ceria (CeO₂), doped bismuth oxide (Bi₂O₃), doped perovskite and amixture thereof, was coated on the substrate 1 through a conventionalslurry coating process or a chemical vapor deposition process prior toperforming a heating treatment, thus finally forming a dense electrolytelayer 2 having a thickness of 10˜50 μm. In such a case, the electrolytelayer 2 was coated on the top surface of the planar portion 31 of thesubstrate 1 and on the external surfaces of the corner and supportportions 32 and 33 of the substrate 1 as shown in FIG. 2a, thuspreventing a direct gas leakage from the porous substrate 1. Therefore,it has been possible to prevent an undesirable reduction in the sealingeffect of a resulting single cell 4 when a sealant was brought intocontact and reacted with the electrode layers 1 and 3.

[0039] As shown in FIG. 2a, an air electrode 3, or a cathode, wasprinted on the top surface of the planar portion 31 of the electrolytelayer 2 using LSM (La_(0.8)Sr_(0.2)MnO₃) powder prior to performing aheat treatment, thus producing a fuel electrode (anode) support typesingle cell 4 for SOFC stacks.

[0040] In order to produce an SOFC stack using the fuel electrodesupport type single cells 4, two metal separating plates, or first andsecond separating plates 8 and 9, were used. The two separating plates 8and 9 respectively had fuel gas channels and oxidizing gas channels,which extended across each other with the single cells 4 beinginterposed between the fuel gas channels and the oxidizing gas channelswithin a resulting stack. In addition, a ceramic insulating plate 10 wasinterposed between two separating plates 8 and 9 of neighboring stackstructures at a position corresponding to an area free from the singlecells 4. Of the two separating plates 8 and 9, the second one 9,interposed between the first one 8 and the insulating plate 10 as shownin FIGS. 3 and 4, had a plurality of pockets or sealing grooves 12 forseating the bent support portions 33 of the single cells 4. When theseparating plates 8 and 9 and the insulating plate 10 were assembledtogether, a small amount of sealant was applied to the junctions betweenthe two separating plates 8 and 9 and the insulating plate 10, thuspreventing a gas leakage from the junctions at an operationaltemperature of the SOFC stack. As best seen in FIG. 4, the sealinggrooves 12 of the second separating plate 8 were sized to have aclearance of 1˜2 mm inside and outside the bent support portion 33 ofeach single cell 4, and were deeper than the inside height of the bentsupport portion 33 by 0.5˜1 mm.

[0041] Therefore, in the case of using single cells 4 of 50×50 mm, thesealing grooves 12 of the second separating plate 9 had an inside widthof 43 mm, an outside width of 53 mm, a groove width of 5 mm, and a depthof about 1.5˜2.5 mm. The thickness of the second separating plate 9 of aresulting SOFC stack was increased in proportion to the depth of thegrooves 12, and so it was necessary to design the second separatingplate 9 to have as small a thickness as possible. While assembling adesired SOFC stack, a sealant was filled within the sealing grooves 12of the second separating plate 9 prior to placing the bent supportportions 33 of the single cells 4 within the grooves 12. Such a stackingprocess was repeated to completely produce a desired SOFC stack.

[0042] In such a case, two conventional collectors, or first and secondcollectors 13 and 14, were used to bring the air electrode 3 and thefuel electrode 1 of each single cell 4 into electric contact with thefirst separating plate 8 of a neighbouring stack structure and thechannel support 7 or the second separating plate 9 of this stackstructure, respectively. A thin gauze of about 50 mesh, made of Ni, wasused as the first collector 13 positioned around the fuel electrode 1 ofeach single cell 4. On the other hand, a porous conductive ceramicplate, having the same component as that of the cathode 3 or ceramicseparating plates of each single cell 4 and being inexpensive incomparison with a meshy gauze made of a noble metal, such as Pt, wasused as the second collector 14 positioned around the air electrode 3.

[0043] In order to accomplish a desired electric contact of the fuelelectrode 1 of each single cell 4 with the gas channels of the firstseparating plate 8, it was necessary to make the gas channel structurethicker than that of an SOFC stack using the conventional planar typesingle cells of FIG. 1a. Such a thick gas channel structure wasaccomplished by making the gas channel area of the first separatingplate 8 thicker than the other areas of the plate 8. It was alsopossible to more simply make the desired thick gas channel structure bysetting a separate channel support having a smaller size than the widthbetween the bent support portions 33 of each single cell 4 within thestack as shown in FIG. 3. In such a case, the gas channels 6 of thefirst separating plate 8 were effectively brought into electric contactwith the fuel electrode 1 of each single cell 4.

[0044] Therefore, in the SOFC stack structure 11 having the fuelelectrode support type single cells 4 of this Example, the separatingplates 8 and 9 are somewhat thicker than those of a conventional SOFCstack structure. However, the SOFC stack structure 11 of this inventionis advantageous in that it has an improved gas sealing effect,effectively relieves the stress due to a difference in coefficient ofthermal expansion, and stably fixes the position of the single cells 4within the SOFC stack.

EXAMPLE 2 A Process of Producing a Air Electrode (Cathode) Support TypeSingle Cell Which is Bent Downwardly at Four Sides, and a Process ofProducing an SOFC Stack Using Such Single Cells

[0045] In order to produce a desired air electrode support type singlecell 4, LSM (La_(0.8)Sr_(0.2)MnO₃) powder was mixed with graphite powderprior to performing conventional forming and heat treating processes,thus forming a porous substrate, or an air electrode layer 3 having aporosity of 40%. In such a case, the porous cathode substrate 3 had ashape shown in FIG. 2b, with a square size of 55×55 mm, a thickness of asquare portion 31 of 1˜2 mm, an outside height of a bent support portion33 of about 3 mm, an inside height of the bent support portion 33 ofabout 1˜2 mm, and a thickness of the bent support portion 33 of about1˜2 mm. Thereafter, an electrolyte layer 2 was formed on the poroussubstrate 3 in the same manner as that described for Example 1. Inaddition, the lower surface of the electrolyte layer 2 was printed witha starting material, which was made by primarily mixing NiO powder withyittria stabilized zirconia power (ZrO₂+8Y₂O₃, 8YSZ) at a weight ratioof 50:50 and by secondarily mixing 20% graphite powder with the powdermixture, in the same manner as that described for Example 1, thusforming a fuel electrode layer 1. An air electrode (cathode) supporttype single cell 4 of FIG. 2b was produced.

[0046] Thereafter, an SOFC stack of FIGS. 3 and 5 was produced using theair electrode support type single cells 4 of FIG. 2b. In this example,the remaining steps of producing the stack were the same as those ofExample 1.

EXAMPLE 3 A Process of Producing a Self-Support (Electrolyte Support)Type Single Cell Which is Bent Downwardly at Four Sides, and a Processof Producing an SOFC Stack Using Such Single Cells

[0047] In order to produce a desired self-support (electrolyte support)type single cell 4, an electrolyte layer 2 having the shape of FIG. 2cwas primarily formed using a starting material, or granule powder havinga size of 20˜30 μm and being selected from doped zirconia (ZrO₂), dopedceria (CeO₂), doped bismuth oxide (Bi₂O₃), doped perovskite and amixture thereof. Thereafter, the electrolyte layer 2 was sintered. Theelectrolyte layer 2 had ultimately a square size of 50×50 mm, athickness of a square portion 31 of 150 μm, an inside height of the bentsupport portion 33 of 2 mm, and a thickness of the bent support portion33 of about 150˜1000 μm. In such a case, it was also possible to makethe thickness of the bent support portion 33 be 150 μm equal to that ofthe square portion or be 1˜2 mm in the same manner as that described forExample 1.

[0048] Thereafter, LSM (La_(0.8)Sr_(0.2)MnO₃) powder, or the material ofthe air electrode (cathode) layer 3, and NiO-YSZ, or the material of thefuel electrode (anode) layer 1, were printed on the upper and lowersurfaces of the electrolyte layer 2 prior to performing a heat treatingprocess, thus forming a desired self-support type single cell 4 for SOFCstacks.

[0049] In addition, an SOFC stack was produced using the self-supporttype single cells 4 of FIG. 2c and two metal separating plates 8 and 9.In such a case, the two metal separating plates 8 and 9 were arrayed ina way such that the fuel gas channels and the oxidizing gas channels ofthe two plates 8 and 9 extended across each other with the single cells4 being interposed between the fuel gas channels and the oxidizing gaschannels. In addition, a ceramic insulating plate 10 was set between twoseparating plates 8 and 9 of neighbouring stack structures 11 at aposition corresponding to an area free from the single cells 4, thusforming a desired SOFC stack. In the SOFC stack of this Example, it hasbeen possible to reduce the thickness (volume) of the insulating plate10 by the thickness of the self-support type single cells thinner thanthe single cells of Example 1. In this example 3, the remaining steps ofproducing the SOFC stack were the same as those of Example 1.

EXAMPLE 4 A Process of Producing an SOFC Stack Without Sealing theInsulating Plate at a Position Corresponding to the Gap Between theSingle Cells

[0050] Since each insulating plate 10 of each stack structure 11 of FIG.3 was brought into contact with the first separating plate 8 of an upperstack structure at its upper surface and with a second separating plate9 at its lower surface, the insulating plate 10 was set within the SOFCstack with a sealant being applied to both sides of the insulating plate10. The insulating plate 10 of Examples 1 to 3 thus accomplished aconstant flow of oxidizing gas in addition to a desired oxidizing gassealing effect during an operation of the SOFC stack at a hightemperature. However, in the SOFC stack, such an oxidizing gas sealingstructure of the insulating plate 10 was not more important than thefuel gas sealing structure provided by the first and second separatingplates 8 and 9. Therefore, even though the oxidizing gas sealingstructure of the insulating plate 10 was formed by sealing the gasmanifold area around the edge of the insulating plate 10, the oxidizinggas sealing structure was free from badly or seriously affecting theoperational performance of a resulting SOFC stack. Therefore, in thisexample 4, the resulting SOFC stack was produced while limitedly sealingboth the edge and the gas manifold area of the insulating plate 10without sealing the cross-shaped gap between the single cells 4. In theSOFC stack of this Example 4, it was easy to separate the plates 8, 9and 10 from each prior to repairing. In this example 4, the remainingsteps of producing the SOFC stack were the same as those of Examples 1,2 and 3.

EXAMPLE 5 A Process of Producing an SOFC Stack With InterconnectedSealing Grooves

[0051] In order to reduce the number of sealing grooves 12 in theelectrode support type single cells of Examples 1 and 2 and in theself-support (electrolyte support) type single cell of Example 3, theSOFC stack of this Example 5 was produced using a second separatingplate 9 with the sealing grooves 12 being interconnected into a singlegroove. In such a second separating plate 9, the interconnected sealinggroove 12 had the same depth as that of Examples 1, 2 and 3, and theportion of the sealing groove 12 around the gas manifolds 15 had thesame width, 5 mm, as that of Examples 1, 2 and 3. In addition, theportion of the interconnected sealing groove 12 around the single cells4 had a width of above 10 mm, and so the resulting second separatingplate 9 of this Example 5 was free from the steps between the sealinggrooves 12 of FIGS. 4, 5 and 6. In this example 5, the remaining stepsof producing the SOFC stack were the same as those of Examples 1, 2, 3and 4.

EXAMPLE 6 A Process of Producing an SOFC Stack With a First SeparatingPlate Integrated With a Channel Support

[0052] A desired SOFC stack of this Example was produced using a firstseparating plate 8 integrated with a channel support 7. The separatingplate 8 integrated with the channel support 7 simplified the process ofproducing the SOFC stacks since it was possible to remove the necessityfor separately forming the first separating plate 8 and the channelsupport 7. In this example 6, the remaining steps of producing the SOFCstack were the same as those of Examples 1, 2, 3, 4 and 5.

EXAMPLE 7 A Process of Producing an SOFC Stack With Both the SecondSeparating Plate and the Insulating Plate Being Made of the SameMaterial

[0053] In the stacks of Examples 1, 2, 3, 4, 5 and 6, it was possible tomade the second separating plate 9 using a nonconductive materialdifferent from the first separating plate 8. Therefore, the SOFC stackof this Example 7 was produced using a second separating plate 9 made ofa ceramic material, free from a reaction with the sealant filled in thesealing grooves 12, rather than an expensive heat resisting metal. Inthis example 7, the remaining steps of producing the SOFC stack were thesame as those of Examples 1, 2, 3, 4, 5 and 6.

EXAMPLE 8 A Process of Producing a Fuel Electrode (Anode) Support TypeSingle Cell Which is Bent Downwardly at Opposite Two Sides, and aProcess of Producing an SOFC Stack Using Such Single Cells

[0054] In order to produce a desired fuel electrode (anode) support typesingle cell 4, NiO powder and yittria stabilized zirconia power(ZrO₂+8Y₂O₃, 8YSZ) were mixed together at a weight ratio of 50:50, thuspreparing a powder mixture. Thereafter, 20% graphite powder was mixedwith the powder mixture, thus forming a starting material for poroussubstrates. A porous substrate, or a fuel electrode layer 1 having aporosity of 40%, was produced using the starting material. In such acase, the porous substrate 1 had a shape with opposite two sides beingbent downwardly as shown in FIG. 7a. That is, the porous substrate 1 wassintered ultimately into a square size of 55×55 mm, a thickness of asquare portion 31 of 1˜2 mm, an outside height of a bent support portion33 of about 3 mm, an inside height of the bent support portion 33 ofabout 1˜2 mm, and a thickness of the bent support portion 33 of about1˜2 mm. Thereafter, an electrolyte, selected from doped zirconia (ZrO₂),doped ceria (CeO₂), doped bismuth oxide (Bi₂O₃), doped perovskite and amixture thereof, was coated on the substrate 1 through a conventionalslurry coating process or a chemical vapor deposition process prior toperforming a heating treatment, thus forming a dense electrolyte layer 2having a thickness of 10˜50 μm. In the same manner as that described forExample 1, a porous air electrode (cathode) 3 was printed on the topsurface of the electrolyte layer 2 prior to performing a heatingtreatment, thus producing a fuel electrode support type single cell 4with opposite two sides being bent downwardly as shown in FIG. 7a.

[0055] A desired SOFC stack was produced using the single cells 4 ofFIG. 7a. In the process of producing the SOFC stack, a separating plate8, with a plurality of sealing grooves 12 having a shape correspondingto the profile of each single cell 4 and extending in parallel to thechannels 6, was used. In this example, the SOFC stack was producedwithout using the separate channel support 7 and the second separatingplate 9 different from the process of Example 1. The SOFC stack of thisExample 8 had the construction shown in FIG. 8 (and referred to FIG. 4).This SOFC stack was reduced in height in comparison with the SOFC stackof FIG. 3 since the stack was free from the separate channel support 7and the second separating plate 9. In this example, the remaining stepsof producing the SOFC stack were the same as those of Example 1.

EXAMPLE 9 A Process of Producing an Air Electrode (Cathode) Support TypeSingle Cell Which is Bent Downwardly at Opposite Two Sides, and aProcess of Producing an SOFC Stack Using Such Single Cells

[0056] In order to produce a desired air electrode (cathode) supporttype single cell 4, LSM (La_(0.8)Sr_(0.2)MnO₃) powder was mixed withgranule graphite powder prior to performing conventional forming andheat treating processes, thus forming a porous substrate, or an airelectrode layer 3 having a porosity of 40%. In such a case, the poroussubstrate 3 had a shape with opposite two sides being bent as shown inFIG. 7b. The porous substrate 3 was sintered ultimately into a squaresize of 55×55 mm, a thickness of a square portion 31 of 1˜2 mm, anoutside height of a bent support portion 33 of about 3 mm, an insideheight of the bent support portion 33 of about 1˜2 mm, and a thicknessof the bent support portion 33 of about 1˜2 mm. Thereafter, anelectrolyte, selected from doped zirconia (ZrO₂), doped ceria (CeO₂),doped bismuth oxide (Bi₂O₃), doped perovskite and a mixture thereof, wascoated on the substrate 3 through a conventional slurry coating processor a chemical vapor deposition process prior to performing a heatingtreatment, thus forming a dense electrolyte layer 2 having a thicknessof 10˜50 μm. In addition, the lower surface of the electrolyte layer 2was printed with a starting material, which was made by primarily mixingNiO powder with yittria stabilized zirconia power (ZrO₂+8Y₂O₃, 8YSZ) ata weight ratio of 50:50 and by secondarily mixing 20% graphite powderwith the powder mixture, through the same printing process as thatdescribed for Example 1. Therefore, a desired air electrode support typesingle cell 4, with opposite two sides being bent downwardly as shown inFIG. 7b, was produced.

[0057] A desired SOFC stack was produced using the single cells 4 ofFIG. 7b. The construction of the SOFC stack of this Example is shown inFIG. 8 (and referred to FIG. 5). In this example, the remaining steps ofproducing the SOFC stack were the same as those of Example 8.

EXAMPLE 10 A Process of Producing a Self-Support (Electrolyte Support)Type Single Cell Which is Bent Downwardly at Opposite Two Sides, and aProcess of Producing an SOFC Stack Using Such Single Cells

[0058] In order to produce a desired self-support (electrolyte support)type single cell 4, an electrolyte substrate 2 having the shape of FIG.7c was primarily formed using a starting material, or granule powderhaving a size of 20˜30 μm and being made of a material selected fromdoped zirconia (ZrO₂), doped ceria (CeO₂), doped bismuth oxide (Bi₂O₃),doped perovskite and a mixture thereof. Thereafter, the above substrate2, bent downwardly at opposite two sides, was sintered. The electrolytesubstrate 2 from the sintering process was prepared ultimately into asquare size of 50×50 mm, a thickness of a square portion 31 of 150 μm,an inside height of the bent support portion 33 of 2 mm, and a thicknessof the bent support portion 33 of about 150˜1000 μm. In such a case, itwas also possible to make the thickness of the bent support portion 33be 150 μm equal to that of the square portion 31 or be 1˜2 mm in thesame manner as that described for Examples 8 and 9. Thereafter, LSM(La_(0.8)Sr_(0.2)MnO₃) powder, or the material of an air electrode(cathode) layer 3, and NiO-YSZ, or the material of a fuel electrode(anode) layer 1, were printed on the upper and lower surfaces of theelectrolyte substrate 2 prior to performing a heat treating process,thus forming a desired self-support (electrolyte support) type singlecell 4 with two porous electrode layers 1 and 3.

[0059] A desired SOFC stack was produced using the single cells 4 ofFIG. 7c. The SOFC stack of this Example had a construction as shown inFIG. 8 (and referred to FIG. 6). In this example, the remaining steps ofproducing the SOFC stack were the same as those of Example 8.

EXAMPLE 11 A Process of Producing an SOFC Stack With an Improved GasChannel Sealing Structure on Both an Insulating Plate and a SeparatingPlate

[0060] In order to improve the sealing effect between the insulatingplate 10 and the separating plate 8 while producing the SOFC stack ofFIG. 8, a small amount of sealant was applied to the upper and lowersurfaces of the plates 8 and 10. However, it was possible for thesealant to flow down into the fuel gas channels with the passage oftime, thus undesirably disturbing a smooth gas flow within the channelor losing a desired sealing effect.

[0061] In an effort to overcome such a problem derived from the stacksof Examples 8, 9 and 10 with the gas channels passing perpendicularly atthe junction between the single cells 4 on the separating plate 8, theSOFC stack of this Example 11 was produced while lowering the gaschannel to a predetermined depth (example, a half depth of channel) witha plurality of flat plates or foot bridge plates having the samethickness of the depth and being positioned on the channels. Therefore,the sealing effect between the separating plate 8 and the insulatingplate 10 of the stack was improved while accomplishing the desiredsmooth gas flow within the gas channels. In this example, the remainingsteps of producing the SOFC stack were the same as those of Examples 8,9 and 10.

EXAMPLE 12 A Process of Producing an SOFC Stack With a Reduced Number ofSealing Grooves Formed on the Separating Plate

[0062] In order to reduce the number of sealing grooves (sealingpockets) on the separating plate 8 in the SOFC stacks of Examples 8, 9,10 and 11, the stack of this Example 12 was produced, with the steps ofthe grooves 12 at positions between the single cells 4 being removed soas to at least partially integrate the sealing grooves 12 into a singlegroove 12. In such a case, the steps were removed from the positionscorresponding to the junctions between the single cells 4 rather thanthe positions around the gas manifolds 15 without changing the depth ofthe sealing groove 12. Therefore, on the separating plate 8, thehorizontally and/or vertically neighboring single cells 4 commonly usedthe sealing groove 12 having a width of 10 mm. In this example, theremaining steps of producing the SOFC stack were the same as those ofExamples 8, 9, 10 and 11.

EXAMPLE 13 A Process of Producing an SOFC Stack With the SealingStructure Being Removed from the Junction Between the Bent SupportPortions of the Single Cells and the Insulating Plate

[0063] In the SOFC stack of FIG. 8, it was possible to remove thecross-shaped portion from the central portion of the insulating plate 10without affecting the functioning of the resulting stack. Therefore, theSOFC stack of this Example was produced using an insulating plate 10,with the cross-shaped portion being removed from the central portion ofthe insulating plate 10 rather than the portions around the bent supportportions 33 of the single cells 4 or the portions around the gasmanifold sealing edge of the insulating plate 10 different from thestacks of Examples 8, 9, 10, 11 and 12. In this example, the remainingsteps of producing the SOFC stack were the same as those of Examples 8,9, 10, 11 and 12.

[0064] As described above, the present invention provides a single celland a stack structure for SOFC stacks. In the single cell, two or fouropposite sides are bent downwardly to form an electrode support typestructure or a self-support (electrolyte support) type structure eachhaving a reversed U-shaped cross-section. The present invention alsoprovides an SOFC stack structure, with such single cells beinggastightly stacked on a separating plate having a plurality of sealinggrooves sealed with sealant. In the SOFC stack of this invention, it ispossible to prevent the fuel gas and the oxidizing gas from being mixedtogether while simply accomplishing a desired gas sealing effect. TheSOFC stack is thus free from the stress due to a difference incoefficient of thermal expansion between the single cells and theseparating plate when the temperature of the stack is raised or lowered.Since the sealant is stably kept within the sealing grooves regardlessof an environmental change, it is possible to operate the SOFC stackwhile raising or lowering the temperature of the stack without affectingthe functioning of the stack. When the SOFC stack of this invention isassembled, the bent support portions of the single cells are preciselyseated in the sealing grooves (sealing pockets) of the separating plate.The single cells are thus stably held within the SOFC stack irrespectiveof external impact or thermal stress. Therefore, the present inventionlengthens the expected life span of the SOFC stack, improves thedurability and the operational reliability of the stack, and allows auser to more easily repair the stack when necessary.

[0065] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A single cell for solid oxide fuel cell stacks,comprising a fuel electrode (anode), an electrolyte and an air electrode(cathode), is shaped while being bent downwardly with opposite two orfour sides of said single cell, thus forming an electrode support typesingle cell or a self-support (electrolyte support) type single celleach having a reversed U-shaped cross-section.
 2. The single cellaccording to claim 1, wherein said fuel electrode (anode) is shapedwhile being bent downwardly to have a planar portion, a corner portionand a bent support portion, with the electrolyte having a densestructure and being coated on a top surface of said planar portion ofthe fuel electrode and on at least a part of external surfaces of saidcorner and bent support portions of the fuel electrode and with the airelectrode having a porous structure and being printed on a top planarsurface of said electrolyte, thus forming a fuel electrode (anode)support type single cell having an at least triple-layered structure. 3.The single cell according to claim 1, wherein said air electrode isshaped while being bent downwardly to have a planar portion, a cornerportion and a bent support portion, with the electrolyte having a densestructure and being coated on a lower surface of said planar portion ofthe fuel electrode and on at least a part of external surfaces of saidcorner and bent support portions of the fuel electrode and with the fuelelectrode having a porous structure and being coated on a lower planarsurface of said electrolyte, thus forming an air electrode (cathode)support type single cell having an at least triple-layered structure. 4.The single cell according to claim 2 or 3, wherein said electrodesubstrate bent downwardly at its opposite two or four sides and a denseelectrolyte layer coated on said electrode substrate prior to beingheat-treated, said electrolyte layer having a thickness of 10˜50 μm andbeing made of a material selected from doped zirconia (ZrO₂), dopedceria (CeO₂), doped bismuth oxide (Bi₂O₃), doped perovskite and amixture thereof, with the air electrode and the fuel electroderespectively coated on upper or lower surfaces of said electrolyte, thusforming an electrode support type single cell having an at leasttriple-layered structure.
 5. The single cell according to claim 1,wherein said electrolyte is shaped while being bent downwardly at itsopposite two or four sides so as to have a planar portion, a cornerportion and a bent support portion, with the fuel electrode (anode)being coated on a lower surface of said planar portion of theelectrolyte and with the air electrode (cathode) being coated on a topsurface of said electrolyte, thus forming a self-support (electrolytesupport) type single cell having an at least triple-layered structure.6. The single cell according to claim 5, wherein the electrolyte isformed as an electrolyte plate made of granule powder, having a size of20˜30 μm and being made of a material selected from doped zirconia(ZrO₂), doped ceria (CeO₂), doped bismuth oxide (Bi₂O₃), dopedperovskite and a mixture thereof, said electrolyte plate having athickness of 150˜1000 μm and being shaped while being bent downwardly atopposite two or four sides thereof, with the air electrode and the fuelelectrode respectively coated on upper and lower surfaces of a planarportion of said electrolyte, thus forming the self-support (electrolytesupport) type single cell having an at least triple-layered structure.7. A solid oxide fuel cell stack structure, comprising: a firstseparating plate having a plurality of gas channels; a second separatingplate having a plurality of sealing grooves and being laid on the firstseparating plate; a plurality of channel supports seated on the gaschannels of the first separating plate at positions within the secondseparating plate; a plurality of solid oxide fuel single cells eachconsisting of a fuel electrode, an electrolyte and an air electrode,with four sides of each single cell being bent downwardly so as to forman electrode support type single cell or a self-support (electorlytesupport) type single cell having a reversed U-shaped cross-section, saidsingle cells being seated on the second separating plate while beingpositioned on said channel supports with bent support portions of thesingle cells being held within the sealing grooves filled with asealant; a first collector positioned between each of said channelsupports and an associated single cell; a second collector positioned oneach of said single cells; and an insulating plate laid on said secondseparating plate.
 8. The solid oxide fuel cell stack structure accordingto claim 7, wherein said first and second separating plates and saidinsulating plate are sealed at their junctions by a sealant at positionsaround gas manifolds while removing a cross-shaped central portion orthe whole of the insulating plate, and so oxidizing gas channels betweenthe single cells are not individually sealed.
 9. The solid oxide fuelcell stack structure according to claim 7, wherein the sealing groovesof said second separating plate are interconnected together at leastpartially, thus allowing the single cells to be commonly held by theinterconnected sealing grooves.
 10. The solid oxide fuel cell stackstructure according to claim 7, wherein said first separating plate isintegrated with said channel support.
 11. A solid oxide fuel cell stackstructure, comprising: a separating plate having a plurality of gaschannels and a plurality of sealing grooves extending in parallel to thegas channels; a plurality of solid oxide fuel single cells eachconsisting of a fuel electrode, an electrolyte and an air electrode,with opposite two sides of each single cell being bent downwardly so asto form an electrode support type single cell or a self-support(electrolyte support) type single cell having a reversed U-shapedcross-section, said single cells being seated on said separating platewith bent support portions of the single cells being held within thesealing grooves filled with a sealant and with a junction between thesingle cells being directly sealed by a sealant on the gas channels; afirst collector positioned between each of said gas channels and anassociated single cell; a second collector positioned on each of saidsingle cells; and an insulating plate laid on said separating plate. 12.The solid oxide fuel cell stack structure according to claim 11, whereinthe single cells are held on the separating plate by planar plates orfoot bridge plates while removing the gas channels of the same width anddepth as those of planar plates or foot bridge plates, thus forming aplurality of fuel gas channels.
 13. The solid oxide fuel cell stackstructure according to claim 11, wherein the sealing grooves of saidseparating plate are interconnected together at least partially, thusallowing the single cells to be commonly held by the interconnectedsealing grooves.
 14. The solid oxide fuel cell stack structure accordingto claim 11, wherein said separating plate and said insulating plate aresealed at their junction by a sealant at positions around gas manifoldswhile removing a cross-shaped central portion or the whole of theinsulating plate, and so oxidizing gas channels between the single cellsare not individually sealed.